Conventionally, an energy storage device is known which is equipped with a positive electrode and a negative electrode to have a structure in which the positive electrode includes a positive active material layer, the negative electrode includes a negative active material layer, and the positive active material layer and the negative active material layer are arranged to face each other.
As such an energy storage device, a nonaqueous electrolyte secondary battery is proposed which is further equipped with a porous layer for electrically insulating positive and negative electrodes from each other, the porous layer being arranged between the positive active material layer and the negative active material layer (WO 2010/050507).
In the energy storage device disclosed in WO 2010/050507, the positive and negative electrodes are electrically insulated from each other by the porous layer.
However, the nonaqueous electrolyte secondary battery disclosed in WO 2010/050507 merely includes the positive and negative electrodes, and the porous layer. Thus, immediately after charge-discharge cycles (for example, within 2 hours of the end of the cycles), the battery may be temporarily decreased in power (hereinafter, the power-decrease may be referred to temporary deterioration).